Method and apparatus for the abatement of toxic gas components from a semiconductor manufacturing process effluent stream

ABSTRACT

An apparatus and process for abating at least one acid or hydride gas component or by-product thereof, from an effluent stream deriving from a semiconductor manufacturing process, comprising, a first sorbent bed material having a high capacity sorbent affinity for the acid or hydride gas component, a second and discreet sorbent bed material having a high capture rate sorbent affinity for the same gas component, and a flow path joining the process in gas flow communication with the sorbent bed materials such that effluent is flowed through the sorbent beds, to reduce the acid or hydride gas component. The first sorbent bed material preferably comprises basic copper carbonate and the second sorbent bed preferably comprises at least one of, CuO, AgO, CoO, CO 3 O 4 , ZnO, MnO 2  and mixtures thereof.

This application is a continuation-in-part of U.S. application Ser. No. 10/314,727, filed on Dec. 9, 2002, now pending.

BACKGROUND

1. Field of the Invention

The present invention relates to a method and apparatus for removing toxic gas components exhausted from a semiconductor process using same.

2. Description of the Related Art

In the manufacture of semiconductors, the toxic, flammable, and corrosive nature of hydride and acid gases pose considerable health and environmental hazards in addition to jeopardizing the integrity of exhaust systems.

Material gases, such as BF₃, AsH₃ and PH₃ are used as primary dopant gases in ion implantation processes. Other gases, such as SiF₄, GeF₄, (hfac)In(CH₃)₂ and Br₂Sb(CH₃) etc. are also used.

In the case of MOVPE/PECVD gases such as SiH₄, SiF₄, NH₃, AsH₃ and PH₃ are delivered to a process chamber through electrically isolated assemblies. AsH₃ or PH₃ are flowed at particularly high rates during deposition to provide atomic As or P of GaAs or GaP, respectively. HCl can be flowed over Ga metal in order to provide GaCl as a precursor to the atomic Ga of GaN. Epitaxial dielectric is deposited from SiH₄.

HCl and HF can be used for chamber cleans by creating radicals in a plasma stream, which flow to areas in the chamber where excess film accumulates. The radicals react with the deposited film to create gaseous by-products. The by-products are then removed from the chamber and pumped out as effluent.

Ongoing research focused on reducing emission levels of such toxic gases from the effluent waste streams of semiconductor manufacturing processes, involves the optimization of abatement processes. Current processes include a variety of thermal, wet and/or dry scrubbing operations.

Thermal scrubbers react an oxidizing agent (almost always air) with a target component (e.g. AsH₃, PH₃, etc.) in a process effluent stream to produce an oxidized species of the target component (e.g. As₂O₃, P₂O₅, etc.). The oxidized species is then removed from the effluent stream by contacting the stream with a gas absorption column (water scrubber). The disadvantages of such a system are (a) it is energy intensive in that it requires significant amounts of electricity and/or fuel, such as H₂ or CH₄, (b) it requires water, and (c) it produces an aqueous hazardous waste stream when it scrubs arsenic containing compounds.

Wet scrubbing of effluent streams involves contacting the effluent gas from a specific process with a scrubbing liquid to cause undesired effluent stream components to be absorbed by the liquid, or to react with the liquid (e.g., a caustic solution for contacting with an acid gas effluent) to effect the removal of the undesired components from the gas phase. Often the scrubbing liquid includes an oxidizing agent such as potassium permanganate, a regulated substance, or sodium hypochlorite, which leads to unwanted precipitation reactions. Further, the wet scrubbing system requires the consumption of significant amounts of the oxidizing agents and leads to a contaminated aqueous waste stream.

Dry scrubbing involves contacting the effluent gas with a solid material which functions to chemisorb or react with the undesired components to effect their removal. Dry scrubbing concentrates and fully contains hazardous abated components, is passive in operation, has no moving parts and works on demand, making it the safest and most preferable mode of abatement operation.

With respect to ion implant processes it is expected that fluorinated acid gases will pass through an ion implant system largely intact, while hydride source gases will pass through only moderately intact. Thus, the large flow-rates of intact acid gas components and the high toxicity and low levels of permissible personnel exposure of hydride gas components (for example, the threshold limit value (TLV) for AsH₃ is 0.05 ppm, or a IDLH of 3 ppm) mandate highly efficient effluent stream treatment and/or abatement for removal of both gas types.

In addition to the foregoing issues incident to the use and operation of ion implantation systems, empirical characterization of ion implant process exhaust streams reveal significant emissions of hazardous gases in the process system from source gas pumps, roughing pumps and from cryogenic pump regeneration cycles.

It is important to note that for dry scrubbing purposes, the chemical requirements to scrub acid gases such as BF₃ and SiF₄ are entirely different than the chemical requirements to scrub hydride gases such as AsH₃, PH₃ and GeH₄.

With respect to chemical vapor deposition (CVD) processes, an acid gas and/or hydride gas may be used in combination with large amounts of a ballast or process gas, (e.g. hydrogen). Dry scrubbing of effluent streams deriving from such a process is difficult because a secondary reaction, typically reductive hydrogenation may occur between the scrubbing material and hydrogen at a temperature around 110° C. to 120° C. The secondary reaction, once initiated, may lead to a “runaway” situation, where temperatures in the scrubbing material reach 600° C.

Cupric oxide CuO, cupric hydroxide Cu(OH)₂, and copper carbonate CuCO₃, based materials are used in resin scrubbing materials for abatement of hydride compounds from semiconductor effluent streams. Although most copper based resins react exothermically with hydrides, CuO and Cu(OH)₂, materials can exotherm severely at temperatures between 110° C. to 120° C. in the presence of large quantities of hydrogen.

For example, U.S. Pat. Nos. 4,743,435, 4,910,001 and 4,996,030 disclose methods for removing hydride gas species from MOCVD applications using CuO based resins. However, the capacity of CuO based materials is limited due to the copper surface area and when attempts are made to increase CuO content, most notably by addition of CuO to a metal oxide mixture used to create the resin, the surface area can drop inordinately. Additionally, the material can exotherm severely due to the heats of adsorption and reaction of the hydride gas and since most CVD applications use large amounts of H₂ as carrier gas with AsH₃ and PH₃, temperature of the CuO material is critical, as exceeding a temperature of approximately 110° C. to 120° C. may result in the reduction of the CuO material by H₂, thereby creating severe exotherms that can exceed 500° C. to 600° C.

U.S. Pat. No. 5,853,678 discloses a method of removing harmful, volatile, inorganic hydrides, inorganic halides and organometallic compounds by contacting the harmful compound with crystalline cupric hydroxide. However, similar to CuO based materials, crystalline cupric hydroxide, Cu(OH)₂, based materials react with hydride gases to undergo H₂ reduction reactions in approximately the same temperature regime as CuO.

Japanese publication JP1996059391A discloses a method of removing Group V species from effluents containing same by contacting the Group V species with basic copper carbonate CUCO₃.Cu(OH)₂. Basic copper carbonate undergoes H₂ reduction reactions at temperatures higher than both CuO and Cu(OH)₂.

It would therefore be a significant advance in the art, and accordingly is an object of the present invention, to provide an abatement system capable of handling acid, hydride and/or metalorganic gases, which eliminates or at least ameliorates the aforementioned hazards of conventional CuO and Cu(OH)₂ processes.

It is another object of the invention to provide an improved system for the treatment of effluent streams comprising acid, hydride and/or metalorganic gaseous components having a potential operating temperature regime that is greater than 100° C.

It is another object of the instant invention to provide a safe, low-cost, high capacity, high efficiency system for the treatment of effluent streams comprising acid, hydride, and/or metalorganic gaseous components.

Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.

SUMMARY

In one aspect, the present invention relates to an apparatus for abatement of at least one toxic gas component or by-product thereof, from an effluent stream deriving from a semiconductor manufacturing process, such apparatus comprising:

-   -   a first sorbent bed material having a high surface area and a         physical sorbent affinity for at least one toxic gas component;     -   a second, discreet, sorbent bed material having a high capacity         sorbent affinity for said the at least one toxic gas component;     -   a third, discreet, sorbent bed material having a high capture         rate sorbent affinity for the at least one toxic gas component;         and     -   a flow path joining the process in gas flow communication with         the sorbent bed materials such that the effluent stream contacts         the first sorbent bed material prior to contacting the second         and third sorbent bed materials to at least partially remove the         at least one toxic gas component from the effluent stream.

In a further aspect, the present invention relates to an apparatus for abatement of at least one acid gas, hydride gas or by-product thereof, from an effluent stream deriving from a semiconductor manufacturing process, such apparatus comprising:

-   -   a first sorbent bed material having a high surface area and a         physical sorbent affinity for at least one acid gas, hydride gas         or by-product thereof;     -   a second, discreet, sorbent bed material having a high capacity         sorbent affinity for the at least one acid gas, hydride gas or         by-product thereof;     -   a third, discreet, sorbent bed material having a high capture         rate sorbent affinity for said at least one acid gas, hydride         gas or by-product thereof; and     -   a flow path joining the process in gas flow communication with         the sorbent bed materials such that the effluent stream contacts         the first sorbent bed material prior to contact the second and         third sorbent bed materials to at least partially remove the at         least one acid gas, hydride gas or by-product thereof from the         effluent stream.

In a further aspect, the present invention relates to a layered dry resin sorbent system for abatement of an acid gas or hydride gas or by-product thereof, comprising:

-   -   a first sorbent bed material having a high surface area and a         physical sorbent affinity for the at least one acid gas, hydride         gas or by-product thereof;     -   a second, discreet, sorbent bed material having a high capacity         sorbent affinity for the at least one acid or hydride gas         component;     -   a third, discreet, sorbent bed material having a high capture         rate sorbent affinity for the at least one acid or hydride gas         component; and     -   a flow path joining the process in gas flow communication with         the sorbent bed materials such that the effluent stream contacts         the first sorbent bed material prior to the second and third         sorbent bed materials to at least partially remove the at least         one acid gas, hydride gas or by-product thereof from the         effluent stream.

In a still further aspect, the present invention relates to a process for reducing the concentration of at least one toxic gas component from a semiconductor process effluent stream containing same, said method comprising:

-   -   contacting the semiconductor process effluent with first,         second, and third, sorbent composition layers, so as to retain a         portion of the toxic gas component on each of the first, second         and third sorbent composition layers;     -   wherein the first layer comprises a material having a high         surface area and a physical sorbent affinity for the toxic gas         component, the second, sorbent layer comprises a material having         a high sorptive capacity for the toxic component and the third         sorbent layer material comprises a material having a high         capture rate sorptive affinity for the toxic component.

Other aspects and features of the invention will be more fully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C show a comparison of different sorbent configurations useful in the instant invention.

FIG. 2 is a schematic representation of a sorbent canister unit in accordance with one embodiment of the present invention.

FIG. 3 is a schematic representation of a chemical vapor deposition scrubber system according to one embodiment of the invention.

FIG. 4 is a schematic representation of a thin film fabrication facility including an effluent abatement system according to one embodiment of the present invention.

DETAILED DESCRIPTION

The present invention relates to an apparatus and method for the abatement of toxic gas components present in an effluent gas stream deriving from a semiconductor manufacturing process. More particularly the present invention relates to an apparatus and method for the abatement of at least one acid or hydride gas component or by-product thereof present in an effluent gas stream from a Group III-V semiconductor manufacturing process.

The present invention further relates to an apparatus and method for the abatement of hydride and acid gas components present in an effluent gas stream deriving from a semiconductor manufacturing process, where extremely high abatement efficiencies are required.

The inventors of the instant invention have discovered that the efficiency of a particular sorbent based abatement system, may be increased by two to four fold, by placing a layer of a high surface area sorbent bed material having a physical sorbent affinity for a toxic gas component, upstream of a of a main bed, high capacity, or high capture rate sorbent material.

In one embodiment, the present invention relates to an apparatus for abatement of at least one toxic gas component or by-product thereof, from an effluent stream deriving from a semiconductor manufacturing process, such apparatus comprising:

-   -   a first sorbent bed material having a high surface area and         physical sorbent affinity for trapping the toxic gas component;     -   a second sorbent bed material having a high capacity sorbent         affinity for the at least one toxic gas component;     -   a third and discreet sorbent bed material having a high capture         rate sorbent affinity for the at least one toxic gas component;         and     -   a flow path joining the process in gas flow communication with         the sorbent bed materials such that the effluent stream contacts         the first sorbent bed material prior to the second and third         sorbent bed materials and at least partially removes the at         least one toxic gas component from the effluent stream.

In a preferred embodiment, the semiconductor manufacturing process is a Group III-V process and the effluent gas stream comprises at least one acid or hydride gas component or byproduct thereof, which contacts the first sorbent bed prior to contacting the second and third sorbent beds.

Semiconductor manufacturing processes frequently employing acid, hydride and/or metalorganic gaseous components include ion implant, metalorganic chemical vapor deposition (MOCVD) and plasma enhanced chemical vapor deposition (PECVD). Typical acid, hydride and/or metalorganic components and process by-products, which are consumed and usefully abated in such processes, include but are not limited to, AsH₃, PH₃, SbH₃, BiH₃, GeH₄, SiH₄, NH₃, HF, HCl, HBr, Cl₂, F₂, Br₂, BCl₃, BF₃, AsCI₃, PCI₃, PF₃, GeF₄, AsF₅, WF₆, SiF₄, SiBr₄, COF₂, OF₂, SO₂, SO₂F₂, SOF₂, WOF4, CIF3 (hfac)In(CH₃)₂H₂As(t-butyl), H₂P(t-butyl), and Br₂Sb(CH₃). Other usefully abated components include chlorosilanes such as SiHCl₃, and SiH₂Cl₂.

In one embodiment, the apparatus of the present invention comprises a layered dry resin sorbent system for abatement of at least one acid gas component, hydride gas component or by-product thereof, from a Group III-V semiconductor process of a semiconductor manufacturing facility, comprising:

-   -   a first sorbent bed material having a high surface area,         physical sorbent affinity for trapping the toxic gas component;     -   a second sorbent bed material having a high capacity sorbent         affinity for the at least one acid gas, hydride gas or         by-product component;     -   a third sorbent bed material having a high capture rate sorbent         affinity for the at; and     -   a flow path joining the process in gas flow communication with         the sorbent bed materials such that effluent is flowed through         the sorbent beds, to at least partially remove the at least one         acid, hydride, or byproduct component from the effluent.

The layered sorbent system of the instant invention comprises at least discrete, first, second and third sorbent layers where the second “slow” layer has a mass transfer zone (MTZ) that is greater than the “fast” third sorbent layer as well as a higher intrinsic or theoretical capacity.

The sorbent materials of the instant invention may comprise a physisorbent and/or a chemisorbent, as desired to effect desired abatement of effluent species.

The following definitions are provided for terms used herein:

-   -   “layer” is defined as a tabular body of sorbent articles, lying         parallel to a supporting surface and distinctly limited above         and below.     -   “TLV” is defined as the threshold limit value of a particular         species and reflects the level of exposure to the particular         species that a typical person can experience without an         unreasonable risk of disease or injury.     -   “TLV sorbent capacity” is defined as the moles of gaseous         component retained by a sorbent material per liter of sorbent         material, when the breakthrough concentration reaches the         species' TLV value;     -   “mass transfer zone” (MTZ) is defined as the length of sorbent         layer required in order to remove a target contaminant (hydride         gas, acid gas and/or byproduct thereof) from its incoming         concentration level to a level defined as the breakthrough level         (i.e. the concentration level at which a sorbent bed requires         change-out, commonly chosen as the TLV of a particular species);     -   “slow” is defined as having a MTZ thickness that is between 25         and 100% of the total thickness of the first sorbent layer; and     -   “fast” is defined as having a MTZ thickness that is less than         the MTZ thickness of the “slow” sorbent layer.

For the “fast” sorbent layer to have a shorter MTZ, the “fast” sorbent layer material must have a “capture rate” for a particular target species that is higher than that of the “slow” layer material. The capture rate is the sum of all rates (e.g. characteristic mass transfer rate, gas adsorption rate, and chemical reaction rate of the target species with the sorbent material).

The MTZ thickness of the “slow” sorbent bed layer is intrinsically thicker than the MTZ of the “fast” sorbent bed layer. The thickness of the MTZ for the second sorbent bed layer is preferably between 10 and 100% of the total thickness of the second and third layers. More preferably, the thickness of the MTZ of the second sorbent bed layer is between 25 and 75% of the total thickness of the second and third layers. Most preferably, the thickness of the MTZ of the second sorbent bed is about 50% of the total thickness of the second and third layers. Under certain steady state conditions the thickness of the MTZ of the “slow” sorbent layer may be greater than 100% of the thickness for the second and third layers, as the thickness of the MTZ will grow with time. The long MTZ advantageously helps to diffuse heat evolution, which occurs from the chemical reaction between the sorbent material and target species.

Layering of the sorbent materials involves the arrangement of placing layers of at least one high surface area trapping adsorbent upstream of a “slower” high capacity resin material and both the high surface trapping adsorbent and “slower” resin material upstream of at least one layer of a “faster” high capture rate resin material. The “slower”, high capacity resin materials typically require longer mass transfer zones (MTZ). Conversely, the “faster” high capture rate resin materials often have a lower sorbent capacity for the target gas, but have high capture rates (shorter MTZ).

The high surface area trapping adsorbent may comprise a single composition or mixture of discreet components. In one embodiment, the trapping adsorbent comprises a material such as carbon, alumina, silica, diatomaceous earth or zeolite having a high surface area (e.g. between 100 to 1000 m²/g).

In one embodiment, the high surface area trapping adsorbent comprises between 10 and 60% of the total sorbent material used in the system, the “slow” sorbent bed material comprises between 20 and 80% and the “fast” sorbent bed material comprises between 5 and 30% of the total sorbent material used in the system. Preferably the high surface area trapping adsorbent comprises between 40 and 60% of system's total sorbent material.

FIGS. 1A through 1C show a comparison of different sorbent configurations useful in the instant invention. The threshold limit value (TLV) breakthrough for a “slower” high capacity sorbent material occurs before much of the sorbent material is consumed (FIG. 1A). By layering a “faster” high capture rate sorbent material (FIG. 1B), downstream of a “slower” resin material (FIG. 1C), the TLV is shortened and a higher percentage of the “slower” resin material is consumed while achieving high abatement efficiencies.

For the capacity of the “slow” sorbent bed layer to be higher than the capacity of the “fast” sorbent bed layer the first sorbent bed material preferably comprises between 20 to 100 wt % active ingredient.

Preferably, the “slow” sorbent layer comprises a material having a surface area that is less than the surface area of the “fast” sorbent material, making the “slow” sorbent material slower to react with a contaminant species. The surface area of the “slow” sorbent material may range from about 5.0 to 300.0 m²/g). Lower surface areas, and slower capture rates increase the MTZ, which advantageously aids in dispersing heats of reactions.

The MTZ thickness of the “fast” sorbent layer must be less than or equal to the thickness of the MTZ of the “slow” sorbent layer. Such a requirement results in a system having an overall increased capacity. In one embodiment, the MTZ length of the “fast” sorbent layer is less than or equal to 90% of the MTZ thickness of the “slow” sorbent layer, more preferably less than or equal to 50% of the MTZ thickness of the “slow” sorbent layer and most preferably less than or equal to 10% of the MTZ thickness of the “slow” sorbent layer.

The “fast” sorbent material may comprise a single composition or mixture of discreet components. In one embodiment, the “fast” sorbent material comprises a base adsorbent such as carbon, alumina, silica, diatomaceous earth or zeolite having a high surface area (e.g. between 100 to 1000 m2/g) and an active ingredient impregnated into or coated onto a base adsorbent. The amount of active ingredient preferably comprises from about 0.1 to 100% by weight, more preferably between 0.5 and 50% by weight and most preferably between about 1 and 15% by weight of the total weight of the “fast” sorbent material.

The ratio of the percents of “faster” to “slower” sorbent materials useful in the present invention is dependent on the chemical and physical properties of the materials such as surface area, porosity, particles shape, particle size, etc., and may be readily determined by one of ordinary skill in the art. In one embodiment the system of the instant invention comprises a volumetric ratio of “fast” sorbent layer to “slow” sorbent layer of from about 1:20 to 1:1.

The first, second and third layers may comprise sorbent articles of any suitable composition, size, shape and conformation appropriate to the end use application and the specific effluent gas mixture involved in the Group III-V semiconductor process. The sorbent articles may comprise active ingredients and inactive ingredients, and may be in a finely divided form, e.g., beads, spheres, rings, toroidal shapes, irregular shapes, rods, cylinders, flakes, films, cubes, polygonal geometric shapes, sheets, fibers, coils, helices, meshes, sintered porous masses, granules, pellets, tablets, powders, particulates, extrudates, cloth or web form materials, honeycomb matrix monolith, composites (of the sorbent article with other components), or comminuted or crushed forms of the foregoing conformations.

In one embodiment, the sorbent layers comprise particulates having a size range of from about 0.1 mm to 1.5 cm. Preferably, the “slow” sorbent layer comprises particulates having a size range of from about 1.0 mm to 15 mm, more preferably from about 0.8 mm to 7.5 mm, and most preferably from about 1.5 mm to 5.0 mm and preferably the “fast” sorbent layer comprises particulates having a size range of from about 0.1 mm to 7.5 mm, more preferably from about 0.5 mm to 5.0 mm and most preferably from about 0.5 mm to 4.0 mm.

The sorbent layers may be housed in a single containment system or separate containment systems. Preferably, the sorbent layers are housed in a single containment system and the effluent stream comprising acid and/or hydride gas components, contacts the first sorbent layer material prior to contacting the second and third sorbent layer materials.

In one embodiment, the sorbent layers are housed in separate containers and one or more containers comprising at least a first high surface area trapping adsorbent, and a “slow” sorbent layer are coupled to a separate container or polisher comprising at least one “fast” sorbent layer. The containers comprising the high surface area trapping adsorbent and the “slow” sorbent, may comprise same or similar sorbents capable of scrubbing same or differing toxic components from an effluent process stream. Further, the containers comprising the high surface area trapping adsorbent and the “slow” sorbent, may serve the same or different tools. When the high surface area trapping adsorbent and the dual “slow” containers are plumbed to the same process tool, auto-switching capabilities between canisters exists. The polisher may further serve as a back up abatement system for a main abatement apparatus as described herein.

In a further embodiment the overall capacity of the abatement system is maximized by a cyclical process in which a toxic gas species is first flowed into and through the system for a set period of time, (e.g. until the high surface area trapping adsorbent or any other layer becomes ineffective) at which time the ineffective sorbent material is taken off line and regenerated.

The ineffective sorbent material may be regenerated using air, ozone, other oxidizing agent or combinations of the foregoing as well as in combination with N₂ and/or other inert gases. The regeneration step converts chemically or physically trapped toxic gas species (e.g. hydride gas species) into non-volatile solids. Such regeneration provides for reuse of the sorbent material.

In one embodiment, ozone produced by an ozone generator, may be used to regenerate the ineffective sorbent material. More specifically, an ozone generator useful in the present invention may function by passing air between two concentric tubes, where a high voltage is supplied to one tube so as to create an electrical discharge within annular space of the tube. The discharge electrically excites oxygen in the air which recombines to form ozone.

In a further embodiment the capacity of the abatement system may be maximized by cooling of the high surface area physical sorbent material and/or cooling of the effluent gas stream upon contacting the first high surface area physical sorbent material layer.

FIG. 2 is a schematic representation of a sorbent canister unit 200 such as may be employed for the treatment of Group III-V effluent gases in accordance with one embodiment of the present invention.

The sorbent canister unit 200 includes a vessel 202 enclosing an interior vessel volume 204 communicating with the waste gas stream feed passage 206 in inlet 208, and communicating with the scrubbed gas discharge passage 210 in outlet 212. At the respective inlet and outlet ends of the vessel 202 are provided screen or grid members 214 and 216, respectively. These foraminous members serve to contain the bed 211 of sorbent media in the vessel's interior volume, so that solids attrition does not occur in use of the system as waste gas is flowed from the inlet 206 to outlet 212 of the vessel through the bed of sorbent media therein.

The bed 211 of sorbent media may comprise a plurality of discrete zones or layers of different sorbent materials 218 and 220. The sorbent materials 218 and 220 may further comprise different material compositions, which are blended to provide a uniform mixture or monolith of same.

Thus, different sorbent materials are preferably employed, in discrete bed zones or layers wherein such layers may comprise sorbent materials, having different physical affinities for a particular wasted gas species. For example, one such material layer 218 may comprise a high capacity sorbent selective for acid gas components of the effluent gas stream from a process such as ion implantation and/or PECVD, and a second layer 220, may comprise a high capture rate sorbent, also selective for acid gas components in the effluent gas stream.

Further, each layer may comprise a blended mixture of sorbent materials, in which the respective materials are selective for removal of different waste gas species. For example, one such sorbent material may be highly selective for acid gas components of the effluent gas stream, and another sorbent material may be highly selective for hydride gas species in the effluent gas stream.

In a preferred embodiment, the “fast” sorbent layer material comprises a composition selected from the following compositions in approximate weight percents:

-   -   1. copper oxide, (Cu 6%); Silver oxide, (Ag 0.1%); zinc oxide,         (Zn 6.0%); Molybdenum oxide, (Mo 2.5%); triethylenediamine,         (TEDA 3.5%); and activated carbon; and     -   2. manganese oxide, (Mn 22%); copper oxide, (Cu 23%); cobalt         oxide, (Co 10%); silver oxide (Ag 3.5%); and aluminum oxide, (Al         2.6%).

The composition of the high capture rate “fast” sorbents of the present invention are preferably resistant to by-products produced during target species interaction/abatement with the “slow” sorbent layer, including but not limited to H₂O and/or CO₂. Preferably the composition of the “fast” sorbent layer comprises at least one active component selected from the group consisting of, carbon, CuO, Cu₂O, MnOx, wherein x is from 1 to 2 inclusive, AgO, Ag₂O, CoO, CO₃O₄, Cr₂O₃, CrO₃, MoO₂, MoO₃, TiO₂, NiO, LiOH, Ca(OH)₂, CaO, NaOH, KOH, Fe₂O₃, ZnO, Al₂O₃, K₂CO₃, KHCO₃, Na₂CO₃, NaHCO₃, NH₃OH, Sr(OH)₂, HCOONa, BaOH, KMnO₄, SiO₂, ZnO, MgO, Mg(OH)₂, Na₂O₃S₂, triethylenediamine (TEDA) and mixtures thereof. More preferably, the composition of the “fast” sorbent layer comprises at least one component selected from the group consisting of, CuO, AgO, CoO, CO₃O₄, ZnO, MnO₂ and mixtures thereof. In addition to an active component, the composition of the “fast” sorbent layer, may further comprise a stabilizer or the active component may be impregnated into or coated onto an adsorbent substrate.

The composition of the high capture rate “fast” sorbents of the present invention may further comprise a strong base, such as KOH, which increases the abatement kinetics of specific sorbents to acid gases.

In a further embodiment, the present invention relates to a post polishing abatement system comprising a layer of “fast” sorbent material, wherein said layer is positioned downstream from a primary abatement system which may comprise any one of dry, wet, or thermal abatement systems. Preferably the composition of the “fast” sorbent layer comprises at least one component selected from the group consisting of, carbon, CuO, Cu₂O, MnOx, wherein x is from 1 to 2 inclusive, AgO, Ag₂O, CoO, CO₃O₄, Cr₂O₃, CrO₃, MoO₂, MoO₃, TiO₂, NiO, LiOH, Ca(OH)₂, CaO, NaOH, KOH, Fe₂O₃, ZnO, Al₂O₃, K₂CO₃, KHCO₃, Na₂CO₃, NaHCO₃, NH₃OH, Sr(OH)₂, HCOONa, BaOH, KMnO₄, SiO₂, ZnO, MgO, Mg(OH)₂. Na₂O₃S₂, triethylenediamine (TEDA) and mixtures thereof. More preferably, the composition of the “fast” sorbent layer comprises at least one component selected from the group consisting of, CuO, AgO, CoO, CO₃O₄, ZnO, MnO₂ and mixtures thereof.

Table 1 below shows a comparison of dry resin abatement systems with and without a downstream “fast” sorbent polishing system employed. Concentration of target species constituent and linear velocity are identified in Columns 4 and 5 respectively. In all cases, the polishing layer increases the TLV capacity of the target species constituent. The instant invention provides a means by which to increases TLV capacities for particular gaseous components by as much 100% (See Test numbers 12 and 13 below). TABLE 1 Comparison of Various Compositions Useful in the Dual Layer System of the Instant Invention. Polishing Layer Layer 1 “Fast” “Slow” Main Linear Velocity TLV Capacity Test # (Main constituent) Constituents Concentration cm/s mol/L (L/kg) 1 CuO/ZnO/KOH NA 0.8% AsH₃ 4 1.16 (20.0) 2 CuO/ZnO/KOH CuO, MnO₂ 0.8% AsH₃ 4 1.97 (36.4) 3 CuO/ZnO/KOH NA   3% AsH₃ 2 1.89 (21.4) 4 CuO/ZnO/KOH NA   4% AsH₃ 4 1.16 (20.0) 5 CuO/ZnO/KOH NA   2% AsH₃ 4 1.05 (18.1) 6 CuO/ZnO/KOH NA   4% AsH₃ 2 1.60 (27.8) 7 CuO/ZnO/KOH NA   1% AsH₃ 4 0.85 (14.7) 8 CuO/ZnO/KOH CuO, MnO₂   4% AsH₃ 4 1.75 (32.3) 9 CuO/ZnO/KOH NA  50% AsH₃ 0.5  4.6 (79.3) 10 CuO/ZnO/KOH CuO, MnO₂,  50% AsH₃ 0.5 6.0 (110.7) Co₃O₄, AgO 11 CuO/ZnO/KOH NA   1% AsH₃ 1 1.94 (33.2) 12 CuCO₃.Cu(OH)₂ NA   4% AsH₃ 4 1.00 (26.4) 13 CuCO₃.Cu(OH)₂ Activated   4% AsH₃ 4 2.04 (56.7) carbon w/ metals 14 CuCO₃.Cu(OH)₂ Activated  50% AsH₃ 0.5 5.23 (145.4) Carbon w/ metals 15 CuCO₃.Cu(OH)₂ Ni   4% AsH₃ 4 0.94 (26.2) 16 CuCO₃.Cu(OH)₂ CuO, MnO₂,   1% AsH₃ 1 3.67 (102) Co₃O₄, AgO 17 CuCO₃.Cu(OH)₂ Activated   4% (PH₃) 1 3.39 (94.2) Carbon w/ metals 18 CuCO₃.Cu(OH)₂ Activated   4% AsH₃ 4 2.49 (69.2) Carbon w/ metals and TEDA 19 CuCO₃.Cu(OH)₂ NA   2% SiH₄ 1 0.87 (22.9) 20 CuCO₃.Cu(OH)₂ Activated   2% SiH₄ 1 1.17 (34.9) Carbon w/ metals and TEDA 21 CuCO₃.Cu(OH)₂ Activated   1% SiH₄ 1 2.22 (61.4) Carbon w/ metals and TEDA

It will be appreciated that the optimal amount of polish will be based upon the ratio of the mass transfer zone to the overall length of the sorbent beds, (MTZ/LOB). That is, the relative length of the mass transfer zone within the main resin bed will dictate the optimal amount of polish to use.

FIG. 3 shows a plot of TLV capacity as a function of “fast” sorbent polishing layer thickness for an abatement system employing basic copper carbonate, CuCO₃.Cu(OH)₂ as the main bed “slow” sorbent system. As shown, the optimal polishing layer thickness is approximately 25-30% of the total bed length, for a 7 to 9 inches total bed height. As the bed height increases, the optimal polish amount decreases as a fraction of total bed volume.

In a further embodiment the present invention comprises a layered dry resin sorbent system for abatement of an acid gas, hydride gas and/or by-product thereof, from a Group III-V semiconductor process of a semiconductor manufacturing facility, said layered sorbent system comprising, a first layer of “slow” sorbent material having a composition consisting essentially of basic copper carbonate and, a second layer of “fast” sorbent material having a composition consisting essentially of copper oxide, (Cu 6%); Silver oxide, (Ag 0.1%); zinc oxide, (Zn 6.0%); Molybdenum oxide, (Mo 2.5%); triethylenediamine, (TEDA 3.5%); and activated carbon, said sorbent system having a TLV sorbent capacity for arsine at 1 cm/sec of 3.67 moles/liter resin or 102 liters AsH₃/kg resin.

In an alternative embodiment, the present invention comprises a layered dry resin sorbent system for abatement of at least one acid gas component, hydride gas component or by-product thereof, from a Group III-V semiconductor process of a semiconductor manufacturing facility, comprising:

-   -   a first layer of sorbent bed material having a physical sorbent         affinity for said at least one acid gas, hydride gas or         by-product thereof;     -   a second and discreet layer of sorbent bed material having a         high capacity sorbent affinity for said at least one acid gas,         hydride gas or by-product component; and     -   a third and discreet layer of sorbent bed material having a high         capture rate sorbent affinity for said at least one acid gas,         hydride gas or by-product component; and     -   a flow path joining the process in gas flow communication with         the sorbent bed layers such that effluent is flowed through the         sorbent bed layers, to at least partially remove said at least         one acid, hydride, or byproduct component from the effluent.

The sequential flowing of an effluent gas stream comprising acid or hydride gas components through such a layered configuration, results in a system having a higher capacity for removal of gaseous species due to a synergy effect from the physical and chemical adsorbent phenomena of the layered materials.

In such embodiment, the first sorbent bed layer may comprise a high surface area material for trapping or physisorbing the acid or hydride gas component from the effluent stream. Specific examples of useful trapping materials included but are not limited to, with the first layer As a still further alternative, successive vessels may be deployed through which the effluent is sequentially flowed, with the main hydride chemisorbent bed being in a first sorbent vessel, the potassium hydroxide-impregnated material being in a second sorbent vessel, and the hydride chemisorbent, as employed for polishing of the effluent to ensure high hydride removal, being in a third sorbent vessel. Each of the respective sorbent vessels in multi-vessel arrangements of the invention can be of a same or different character, and each bed may be sized and constructed for optimal chemisorption operation, with respect to bed diameter, bed height, particle size, bed void volume, allowable pressure drop, temperature, pressure, flow rate, etc., by design techniques well-known to those skilled in the chemisorbent and effluent abatement art.

Such a configuration effectively utilizes synergistic effects of both physical and chemical phenomena.

In a further embodiment, a system of the present invention may comprise two beds in parallel, wherein each bed comprises a first layer of sorbent bed material having a high surface area physical sorbent affinity for a toxic gas component; a second and discreet layer of sorbent bed material having a high capacity sorbent affinity for said the toxic gas component; and a third and discreet layer of sorbent bed material having a high capture rate sorbent affinity for the toxic gas component. While one bed is active and contacted with toxic gas component, the second bed is regenerated with an oxidizing species, as discussed hereinabove. The thickness of the high surface area sorbent layer may be determined based on the frequency at which the regeneration step occurs.

In a further embodiment the present invention relates to a process for reducing the concentration of at least one hydride gas, acid gas or by-product species from a Group III-V semiconductor process effluent stream containing same, said method comprising:

-   -   contacting said semiconductor process effluent with a first         sorbent layer composition so as to retain at least a portion of         the gaseous species therein; and     -   contacting said process effluent with a second layer sorbent         composition, so as to retain a second portion of the gaseous         species therein,     -   wherein said first sorbent layer comprises a material having a         high sorptive capacity for said gaseous species and said second         sorbent layer material comprises a material having a high         capture rate sorptive affinity for the gaseous species.         Preferably, the process enables a system having a capacity to         retain a particular gaseous species greater than the sum of the         capacities of the individual sorbent layers.

More specifically, the present invention relates to a method of decreasing the concentration of at least one hydride or acid gas species from a semiconductor manufacturing process effluent gas stream comprising same, by contacting the at least one gaseous component with a first solid phase sorbent material comprising at least one of copper oxide, copper hydroxide and basic copper carbonate so as to produce an effluent gas stream having a reduced concentration of gaseous species; and contacting said effluent gas stream having a reduced concentration of gaseous species with a second solid phase sorbent material comprising at least copper oxide, so as to produce an effluent gas stream having a concentration of gaseous species that is less than the TLV for the at least one hydride or acid gas species (e.g. 50 ppb for arsine).

In a further embodiment the present invention relates to a high capacity system for Group III-V process abatement management comprising dual canisters, wherein one canister is online and one is offline and available for auto-switching when canister one reaches a predetermined capacity, said system comprising a layered dry resin sorbent system for abatement of an acid gas, hydride gas and/or by-product thereof, from a Group III-V semiconductor process of a semiconductor manufacturing facility, said layered sorbent system comprising, a first layer of “slow” sorbent material and, a second layer of “fast” sorbent material.

The present invention provides for multi-tool effluent abatement of Group III-V processes, wherein from one to four process tools are combined into a single abatement system for removal of gaseous Group III-V species from a Group III-V semiconductor process effluent stream.

For ease of reference in the ensuing discussion, a generalized description is set out below of FIG. 4, showing a schematic representation of a thin film fabrication facility 400 including an effluent abatement system 402 according to one embodiment of the present invention.

As used herein, the sorbent compositions herein disclosed are intended to be broadly construed, and may alternatively comprise, consist, or consist essentially of the specific stated components or ingredients hereafter specifically identified for such compositions. It will also be understood that such compositions may if desired be devoid of components or ingredients not herein expressly identified.

The system accommodates the collection of un-utilized acid and hydride gases and their by-products in an effluent gas stream by irreversible chemisorption, by contacting the effluent gas stream with a chemisorbent composition, and the continuous monitoring of the chemisorbent scrubber composition to determine the approach to exhaustion of the capacity of the chemisorbent scrubber composition to remove undesired components of the effluent gas stream.

The chemical vapor deposition (CVD) system includes a feed source 404 of source gas for chemical vapor deposition operation, which may comprise a source gas storage and dispensing vessel containing a deposition gas such as AsH₃ (not shown). From the vessel, gas source flows in line 406 to the gas dispensing manifold assembly 408 including line 424, through which the dispensed gas is flowed to the chemical vapor deposition apparatus 426. Concurrently, if desired, a carrier gas, such as argon, helium, hydrogen, etc., is flowed from the carrier gas source 420 in line 422 to the dispensing manifold assembly 408. The carrier gas thus entrains the gas source and forms a gas source mixture.

The gas source mixture flows in line 424 from dispensing manifold assembly 408 to the CVD reactor 426, being dispersed in the interior volume of the reactor. This gaseous source mixture thereby is directed to and impinges on a wafer substrate 432 positioned on susceptor 434 heated by heating element 436. The wafer substrate is maintained at a sufficiently elevated temperature to cause decomposition of the precursor vapor mixture and chemical vapor deposition of the desired component of the precursor on the surface of the wafer substrate.

An effluent comprising waste gases and decomposition by-products is discharged from the reactor 426 and flows in line 438 to the effluent abatement system 402, comprising a layered sorbent system 444, of sorbent materials which may include physical adsorbent(s) and/or chemisorbent(s), for removal therein of contaminant(s) and discharge of a purified effluent gas stream in line 446 to downstream processing or final disposition of same.

Effluent gas from the chemical vapor deposition reactor 426 may also be discharged into a second line 468 and flowed to the abatement system 402 comprising a second layered sorbent system 452, for removal of undesired gas stream components therein, to yield a purified gas stream which is discharged from the abatement system in line 448 and passed to further treatment or other disposition steps.

The abatement system 402 may be provided in duplicate as shown, with one of the systems being a backup scrubbing and/or by-pass unit, and with the lines 438 and 468 containing suitable valving and instrumentation to accommodate such redundancy function, so that one of the beds is initially on-stream and actively scrubbing the effluent gas stream from the chemical vapor deposition reactor 426, while the other is in stand-by mode.

At the outlet end of the sorbent beds 444 and 452, effluent tap lines 460 and 464, which conveys a side-stream of the effluent gas to an endpoint detector 462, as hereinafter more fully described.

The endpoint detector 462, upon detecting the breakthrough or incipient breakthrough of one or more species in the effluent being monitored, generates a control signal that is transmitted in transmission line 464 to the central processing unit (CPU) 466, which may be operatively arranged to effect a corresponding action in the facility.

For example, the system may be arranged so that on incipient breakthrough detection by the endpoint detector, the effluent flow is diverted to a second sorbent bed, or to a holding (surge) vessel for retention until the regeneration of the sorbent bed has been carried out.

Alternatively, or additionally, such endpoint breakthrough detection may be employed to initiate a regeneration sequence, to renew the exhausted sorbent bed for subsequent on-stream operation.

Alternatively, the two scrubber beds 444 and 452 may be concurrently operated, and may each process different effluent streams generated in the operation of the chemical vapor deposition apparatus. For example, one of such abatement systems may process a main effluent gas stream from the chemical vapor deposition apparatus, while the other may for example process a minor effluent stream deriving from pump leakage gas in the effluent treatment system.

The abatement system may be deployed as a separate and distinct apparatus component of the overall system, in relation to the chemical vapor deposition apparatus, and the feed source of the chemical vapor deposition apparatus.

By the system shown in FIG. 4, the process gases leaving the chemical vapor deposition apparatus (e.g., the chemical vapor deposition chamber), are exhausted to vessel(s) containing at least two dry scrubbing sorbent compositions in discrete layers, specific to the chemical vapor deposition gases to be abated in the effluent gas stream. The dry scrubbing compositions remove the waste gases from effluent, (e.g., the chemical vapor deposition chamber exhaust), by chemisorption, irreversibly bonding the waste gas species to the scrubbing medium to maximum operator safety and environmental acceptability of the finally discharged process effluent after its scrubbing treatment.

The features and layout of the semiconductor manufacturing facility shown in FIG. 1 are illustrative in character only, and any other suitable features, arrangements and operation may be advantageously employed.

The dry scrubbing sorbent compositions may therefore be provided in canisters, which are deployed in close proximity to treat the effluent gas stream(s) produced by the process and yield an environmentally acceptable discharged stream. Such canisters can be readily changed out by decoupling same from connecting piping and valving employing conventional connector devices, and replacing the canister of spent sorbent medium with a corresponding canister of fresh medium.

The amount of scrubbing medium used in such disposable canisters for waste gas treatment will be determined by the available dead volume within a particular process tool. With respect to chemical vapor deposition and/or ion implantation, when the canisters are interiorly positioned in the system housing or exteriorly positioned in close proximity to the system housing, the amount of scrubbing medium used will be determined by the deliverable capacity of the feed source of doping gas for the particular system. The feed source volume may be desirably matched to the system throughput, so that the capacity of the source gas vessel does not exceed the removal capacity of the waste gas abatement canister(s) deployed in the system.

Although illustratively shown as comprising a two canister unit, the abatement system may in fact be a multiple bed arrangement comprising multiple sorbent beds variously connected in series, layers, bypass and/or parallel (manifolded) arrangements.

The features, aspects, and advantages of the present invention are more fully shown with reference to the following non-limiting example.

EXAMPLE

A refers to “slow” sorbent bed material and “P” refers to high surface area trapping adsorbent when preceding “A” and “fast” polishing resin material when following “A”.

Experimental Setup:

-   -   a) All tests were performed using a test cell having dimensions         2 inch OD (1.87 inch ID) and approximately 12-inch height.     -   b) The test cell material was stainless steel 304. The cell was         insulated with a layer of 1 inch thick fiberglass.     -   c) To monitor temperature, a 5-pt thermocouple was placed down         the center of the cell.     -   d) The total adsorbent bed height was always 8 inches.     -   e) The ballast gas stream was a mixture containing H₂ and N₂ in         a proportion of 80%/20%, respectively.     -   f) All tests were performed using arsine gas.     -   g) When more than one adsorbent was used, it was layered and not         mixed.

h) In the Table below, the adsorbent order reflects the pathway in which the gas passes through the bed. TABLE 2 Comparison of Various Compositions Useful in the Multi Layer System of the Instant Invention. Adsorbent Test AsH3 Gas Linear Capacity Test # Class Adsorbent Concentration (%) Velocity (cm/sec) (moles/liter) 1 I 100% A 2 2 2.18 2 II  80% A, 20% P 2 2 4.65 3 II  80% A, 20% P 2 2 5.54 4 II  80% A, 20% P 2 2 5.71 5 II  80% A, 20% P 2 2 5.02 6 II  80% A, 20% P 2 2 5.54 7 II  80% A, 20% P 2 2 5.54 8 II  80% A, 20% P 2 2 4.98 9 II  80% A, 20% P 3 2 5.26 10 II  80% A, 20% P 3 1 4.66 11 II  80% A, 20% P 4 1 3.94 12 II  80% A, 20% P 1 2 4.43 13 III 100% P 1 1 0.79 14 III 100% P 2 1 0.97 15 III 100% P 4 1 1.15 16 III 100% P 2 2 0.83 17 IV 100% P 2 2 4.30 18 IV  80% A, 20% P 3 1 7.64 19 V 100% P 2 2 4.33 20 V  50% P, 30% A, 20% P 2 2 7.62

Results

Test No. 1; Test Class I, shows using 100% “A” gives poor results due to the long mass transfer zone of the material.

-   -   Test No.s 2-12; Test Class II, shows replacing the top 20% of         “A” with “P” yields improved results for a variety of conditions         where “P” effectively scavenges small concentrations of         bleed-through from layer “A”.     -   Test No.s 13-16; Test Class III, shows poor results using 100%         “P”, due to low intrinsic capacity of “P”.

Test No.s 17-18; Test Class IV were performed using the following procedure:

-   -   (a) arsine run until TLV breakthrough;     -   (b) adsorbent is regenerated using a stream containing about 4%         O₂ in N₂ (although the O₂ concentration could be lower or         higher);     -   (c) after regeneration, arsine is again flowed to the adsorbent         until TLV break-through is achieved.

The cycle is repeated until the adsorbent has a small fraction of its original (1^(st) cycle) capacity. Regeneration works by driving an intermediate compound (e.g. Cu₃As) to As₂O₃ and the metal oxide. The metal oxide can then re-react with arsine during the subsequent cycle.

Test No.s 19-20; Class V shows a variation of the instant invention where instead of allowing each cycle to proceed to TLV breakthrough, the adsorbent is regenerated after a shorter time period, (e.g. 10-50% of the estimated TLV break-through time), which allows for repeated use of material in the first half of the bed.

It is believed that arsine flowing into the bed, physically adsorbs on the surface (in the first half of the bed), which during air oxidation (regeneration), converts to As₂O₃. When the first half of the bed acts as a physical adsorbent, some concentration of arsine passes onto the second half of the bed. For this reason, the second half of the bed is composed of a high capacity material (e.g. material “A”) followed by fast polishing agent (e.g. material “P”).

Test 19 (adsorbent was 100% “P”) shows similar results to Test 17, where TLV break-through was achieved in each cycle).

Test 20 shows improved results, approximately the same as Test 18, by adding 30% “A” to the second half of the bed. However the cost of the adsorbent bed as a whole decreases significantly because material “A” (and others like it) may cost/liter around $30, whereas “P” may cost approximately $10/liter. The economics are further expanded upon in the following non-limiting example.

Test 18:

-   Cost of Adsorbent=0.8*30+0.2*10=$26/liter -   Cost to treat arsine=$26/liter/7.64 moles arsine/liter=$3.40/mol     arsine

Test 20

-   Cost of Adsorbent=0.3*30+0.7*10=$16/liter -   Cost to treat arsine=$16/liter/7.62 moles arsine/liter=$2.10/mol     arsine -   This method may reduce cost by 38%.

While the invention has been described herein with reference to specific features and illustrative embodiments, it will be recognized that the utility of the invention is not thus limited, but rather extends to and encompasses other features, modifications and alternative embodiments as will readily suggest themselves to those of ordinary skill in the art based on the disclosure and illustrative teachings herein. The claims that follow are therefore to be construed and interpreted as including all such features, modifications and alternative embodiments within their spirit and scope. 

1. An apparatus for abatement of at least one toxic gas component or by-product thereof, from an effluent stream deriving from a semiconductor manufacturing process, such apparatus comprising: a first sorbent bed material having a high capacity sorbent affinity for said at least one toxic gas component; a second and discreet sorbent bed material having a high capture rate for said at least one toxic gas component; and a flow path joining the process in gas flow communication with the sorbent bed materials such that the effluent stream contacts the first sorbent bed material then the second sorbent bed material to at least partially remove the at least one toxic gas component from the effluent stream.
 2. A layered dry resin sorbent system for abatement of an acid gas or hydride gas or by-product thereof, comprising: a first sorbent bed material having a high capacity sorbent affinity for said at least one acid or hydride gas component; a second and discreet sorbent bed material having a high capture rate sorbent affinity for said at least one acid or hydride gas component; and a flow path joining the process in gas flow communication with the sorbent bed materials such that the effluent stream contacts the first sorbent bed material then the second sorbent bed material to at least partially remove the at least one acid gas, hydride gas or by-
 3. A process for reducing the concentration of at least one toxic gas component from a semiconductor process effluent stream containing same, said method comprising: contacting said semiconductor process effluent with a first sorbent layer composition so as to retain at least a portion of the toxic gas component therein; and contacting said process effluent with a second layer sorbent composition, so as to retain a second portion of the toxic gas component therein, wherein said first sorbent layer comprises a material having a high sorptive capacity for said toxic component and said second sorbent layer material comprises a material having a high capture rate sorptive affinity for the toxic component. 